A membrane separates materials based on size of molecules or repellency between molecules and the membrane, and drive force of separation includes pressure, concentration, potential difference, and the like. Advantageously, a membrane allows easy automation and does not involve phase change, high temperature treatment, and the like when used in a separation process, and thus has been studied and utilized as a technique capable of replacing conventional separation processes in environmental pollution prevention facilities or chemical plants.
Examples of such a membrane include a reverse osmosis membrane, a nano-filtration membrane, an ultrafiltration membrane, a microfiltration membrane, an ion exchange membrane, a gas separation membrane, a pervaporation membrane, and the like. Membrane modules are divided into plate-frame type, tubular type, spiral-wound type, and hollow-fiber type membrane modules according to the structure thereof.
Thereamong, the hollow fiber membrane module refers to a bundle of hollow fiber membranes having a shape of a long hollow thread, and includes an inside-out hollow fiber membrane module wherein filtration takes place from inside (lumen) to outside of the hollow fiber membranes and an outside-in hollow fiber membrane module wherein filtration takes place from outside to inside of the hollow fiber membranes. Further, the hollow fiber membrane modules are divided into a pressurized type and a submerged type. The pressurized type hollow fiber membrane module has a structure wherein hollow fiber membranes are placed within a pressure vessel and modularized, and is operated in an outside-in or inside-out mode unlike the submerged type hollow fiber membrane module typically operated in a suction mode.
As shown in FIG. 1, the pressurized hollow fiber membrane module may be divided into a one end-water collection module as shown in FIG. 1(a), a both end-water collection module as shown in FIG. 1(b), and a module which actually allows treated water to be collected at both ends and is provided with an internal flow path for certain reasons, such as elimination of a need for additional piping as shown in FIG. 1(c) according to the number of treated water outlets.
The membrane suffers from deterioration in performance due to membrane fouling caused by adherence of particulates or dissolved substances to a surface of the membrane or surfaces of fine pores. In order to prevent membrane fouling, the membrane is subjected to periodic surface cleaning or backwashing so as to recover performance of the membrane during membrane filtration. Here, backwashing is performed, for example, by passing washing water through the membrane pores in a direction opposite a filtration direction. As backwashing water, filtered water or a mixture of filtered water and acids, alkalis, or cleaning agents such as inorganic and organic detergents is used. Backwashing is performed by pushing the backwashing water through a backwash pump.
In all of the pressurized hollow fiber membrane modules shown in FIG. 1, since hollow fiber membranes are placed in a long cylindrical housing in a longitudinal direction of the module, backwashing pressure is decreased in the longitudinal direction of the hollow fiber membranes during backwashing, thereby causing non-uniform distribution of pressure. In addition, even though the backwashing pressure is uniformly distributed in the longitudinal direction of the hollow fiber membranes, recovery of filtration performance of the membrane can be achieved in a non-uniform manner in the longitudinal direction of the hollow fiber membranes, since a surface layer of the hollow fiber membranes more adjacent to a backwashing water inlet contacts more backwashing water in actual backwashing. Thus, a typical one way-backwashing process has a problem of non-uniform backwashing in the longitudinal direction of the hollow fiber membranes.